chemistry - topic 2: elements, compounds, and mixtures

Who made the first periodic table?

(Dmitri) Mendeleev

How are elements arranged in the periodic table?

Elements are arranged in rows in order of increasing atomic number, and elements with similar properties are arranged in groups.

What groups are most likely to form ions?

1 & 2 (metals) and 6 & 7 (non-metals/halogens)

In what groups do the elements get more reactive as you go down, and why?

Group 1 and 2 - As they gain an electron shell, it gets easier to remove the outer electron to form ions.

In what groups do the elements get less reactive as you go down, and why?

Group 6 and 7 - As they gain an electron shell, it gets harder for the nucleus to attract extra electrons to form ions.

What is an ionic bond?

When a metal and non-metal react together: the metal loses an electron and the non-metal gains an electron, meaning there’s strong electrostatic forces between them — forming an ionic bond.

What is a dot-and-cross diagram, and what does it show?

Dot-and-cross diagrams show bonding and how the atoms react with each other, presented as a dot or a cross.

What structure do ionic compounds have?

A giant ionic lattice.

What are some properties of ionic compounds?

1. High melting and boiling points (due to the strong attraction between the ions).

2. Solid ionic compounds do not conduct electricity, but when it melts, it’ll carry an electric charge.

3. Many dissolve easily in water.

What is a covalent bond?

When non-metals react together: they form covalent bonds by sharing pairs of electrons.

What structure do covalently-bonded substances have?

A simple molecular structure

What are the properties of simple covalent compounds?

1. Covalent bonds are strong because there’s strong electrostatic attractions between the positive nuclei and negative electrons.

2. Forces of attraction between molecules are very weak.

3. Melting and boiling points are very low.

4. Simple molecular substances don’t conduct electricity because they don’t have any free electrons or ions.

Finish the statement:

B___ and S___ models show how the atoms in covalent molecules are connected. They’re great for visualising the structure of molecules in 3D.

Ball and Stick

What atom is a giant covalent structure made from, and what are some examples of the types of molecules it can form?

Carbon: Examples include diamond, graphite, graphene, and fullerenes.

What are some properties of giant covalent structures?

1. No charged ions.

2. Bonded by strong covalent bonds, which means they have high melting and boiling points.

3. Do not conduct electricity, even when molten (except for graphite, graphene and fullerenes).

What are fullerenes and some of their properties?

Another form of carbon in the shape of hollow balls or tubes. They have delocalised electrons, so they can conduct electricity. Their melting points and boiling points aren’t anywhere as high as graphite or diamond, but they’re pretty high for molecular substances because they’re big molecules. They are also nanoparticles.

What is the equation for surface area to volume ratio?

SA to volume ratio = surface area / volume

What is a nanoparticle?

A small particle that ranges between 1-100 nanometres in size.

What are two disadvantages to the use of nanoparticles?

• We’re not sure of the short/long-term impacts on human health.

• Nanoparticles can circulate the body and get into cells due to their small size, which could lead to oxidative damage and even cell death.

What are polymers?

Polymers are formed when monomers join together, which requires high pressure and a catalyst. Polymers are usually carbon based.

What are properties of metals?

• High melting and boiling points

• High density

• Strong, bendy, and malleable

• Good conductors of heat and electricity

What forms when a metal reacts with oxygen?

A metal oxide.

What forms when a metal is mixed with another element?

An alloy.

What structure does a metal have?

A closely-packed crystal structure.

What type of structure has the lowest melting/boiling point?

A simple covalent structure, due to it’s weak intermolecular forces.

What does the term ‘pure’ define as in chemistry?

A substance completely made up of a single element or compound.

Describe the test for purity using boiling and melting points.

Pure substances have specific melting points and boiling points, e.g. pure ice melts at 0°C and pure water boils at 100°C. If a substance is impure, the melting point will be too low, and the boiling point will be too high.

What is simple distillation?

Simple distillation is used for separating a liquid from a solution, for example separating pure water and seawater. Using heat and a distillation flask, it’ll evaporate the part of the solution with the lowest boiling point. The vapour will pass into the condenser and cool and condense, turning it back into a liquid, and collects in the beaker.

What is a disadvantage of simple distillation?

You can only separate things with very different boiling points.

What is fractional distillation?

Fractional distillation is used to separate a mixture of liquids. With a fractionating column, gradually heat the flask. The liquid with the lowest boiling point will evaporate first, and when the temperature on the thermometer matches the BP of the liquid, it’ll reach the top of the column. Liquids with higher BPs may begin to evaporate, however the column is cooler towards the top, so it’ll condense and run back down towards the distillation flask. When the first liquid has been collected, raise the temperature until the next one reaches the top, and repeat.

What is filtration?

The separation of an insoluble solid and a liquid.

What is crystallisation?

The separation of a soluble solid and a solution, via crystallising the solution.

What is chromatography?

Chromatography is the method of separating or identifying substances in a mixture.

What are the two stages or chromatography?

1. The mobile phase - where the molecules can move, which is always a liquid or a gas.

2. The stationary phase - where the molecules can’t move, which is usually a solid or a really thick liquid.

In Thin-Layer Chromatography, what form does the mobile phase take?

A solvent: molecules with a higher solubility in the solvent will spend more time in the mobile phase than the stationary phase - so they’ll be carried further up the ‘plate’.

What is gas chromatography, and what are the mobile/stationary phases?

GC is used to analyse unknown gases. The mobile phase is an un-reactive gas, and the stationary phase is a thick liquid, such as an oil.

What is the relative formula mass of a compound?

The relative atomic masses of each element in the formula added together.

What is an empirical formula?

The simplest whole-number ratio of atoms in the compound. For example, Ethane (C2H6) has the empirical formula CH3.